Of all the artificial X-ray sources, medical radiation sources supply the largest dose to the human population, and of these, Computed Tomography (CT) contributes to 70% of the total dose. A CT examination gives 10-50 times more dose to a patient than corresponding conventional examinations, and therefore quality control is important and should be made regularly.
The absorbed dose to a patient is the result of both primary radiation and scattered radiation from the surrounding tissue. For quality assurance, measurements of the dose from CT are performed in a phantom in order to include internal scattered radiation within the body. Dose measurements are traditionally done with a 100-mm-long, pencil-shaped ionisation chamber. However, at present a CT with beam collimation of up to 160 mm is available, which results in an incomplete measurement of the primary radiation using the standard ion chamber. Measuring the dose profile can be done using thermo luminescent dosimeters (TLD), optically stimulated luminescence (OSL), or X-ray film [3]. Drawbacks of these methods are that they are either expensive, time consuming or obsolete.
Recently, it has been proposed the use of a semiconductor detector diode for point dose measurement. Semiconductor X-ray detectors are as such well known and generally comprise a relatively flat detecting portion with back and front electrode contacts arranged on opposite sides of the detecting portion. The semiconductor detector proposed is made of Si and arranged in a PMMA (poly methyl methacrylate) rod that in turn is arranged in an aluminium cylinder. This type of detection device has e.g. the potential of having a much higher sensitivity than a small ion chamber.
Commercially available solid-state detectors for point dose measurements are typically mounted in a package with a high Z material acting as the back contact causing unsymmetrical response, which is a major drawback in a CT dose profile application. As described by Herrnsdorf et al. (L. Herrnsdorf et al., Nucl. Instr. and Meth. A (2009), doi:10.1016/j.nima.2009.03.159), substituting this high Z back contact with aluminium back and front contacts, and mounting the detector diode in parallel to the incoming radiation, improved the device considerably with regard to dose and dose profile measurements.
Although this improved detection device works reasonably well it is still a need for improvements, it is for instance desired to further improve the symmetry of the response and to further reduce the angle dependency.